According to the U.S. State Department, the population of Macau is 508,000, and the area is 28.2 square kilometers. That makes Macau one of the most densely populated regions on earth. Of that area, 8.9 square miles are on a peninsula, connected to mainland China, and the rest Taipa island, which connects to the peninsula via three bridges.
There exist many not-so-old maps of Macau showing two major islands, Taipa and Coloane. These maps were created before a land reclamation project filled in the gap between, creating the new Cotai section. Many new casino projects are going up on the new Cotai Strip, with some predicting it will be like the Las Vegas Strip of Asia. The Venetian Macao (their spelling) will be the first to open on the Cotai Strip, on August 28, 2008. So if you have one of these old maps, go buy a new one, lest you get very confused when you get there.
That vast majority of the population, tourist attractions, hotels, and casinos are on the peninsula. This is the traditional part of Macau, which I recommend spending most of your time in. Most of Taipa Island is new, and in my opinion, not as interesting. However, there are tons of construction cranes and new development on Taipa. In future years I think it will have much more to offer.